updated sequence selector, set excerpt length to 500

...d/src/main/java/de/vipra/rest/resource/TopicResource.java

vipra-ui/app/html/directives/article-link.html

vipra-ui/app/html/directives/sequence-dropdown.html

vipra-ui/app/html/explorer.html

vipra-ui/app/html/topics/show.html

vipra-ui/app/js/controllers.js

vipra-ui/app/js/directives.js

vipra-ui/app/less/app.less

vipra-util/src/main/java/de/vipra/util/Constants.java